1. Field of the Invention
This invention relates to high intensity vapor arc lamps of the kind comprising an arc discharge tube contained within an outer vitreous envelope, and more particularly to means for securing the discharge tube in position within the outer envelope.
2. Description of the Related Art
High intensity vapor arc lamps, particularly those employing high-pressure sodium and mercury vapor as the arcing medium, are widely used in highway and other outdoor lighting applications because of their efficient luminosity, generally exceeding 100 lumens per watt. In such lamps the vapor and the thermionic electrodes between which the arc is produced are contained within a sealed light-transmissive alumina ceramic tube, alumina being able to withstand the corrosive action of high temperature sodium. In order to conserve heat, which is necessary for efficient operation, the arc tube is axially supported within an evacuated outer elongated vitreous envelope such as glass. The base of the outer envelope terminates in a narrower neck portion which is sealed by a re-entrant stem capped by a press which supports heavy inlead conductors, the inlead conductors being connected to the shell and center contact of a conventional lamp screw base affixed to the neck portion of the envelope. The arc tube is generally supported by a frame which extends longitudinally within the outer envelope, the support frame itself being supported at its lower end by one of the inlead conductors in the base assembly of the outer envelope. In order to further stabilize the support frame, in most commercial arc discharge lamps the upper end thereof is affixed by a clamp or clip to a projecting dimple which is formed in the central region of the dome at the top of the outer envelope. Such designs are disclosed, for example, in U.S. Pat. Nos. 3,906,272 and 3,384,798. It is also known, in lieu of securing the upper end of the support frame to a dimple at the dome of the outer envelope, to affix a plurality of laterally extending leaf springs at several positions along the length of the frame which bear against the inner sides of the envelope. Such a support frame is disclosed, for example, in U.S. Pat. No. 3,767,965.
It has been generally assumed that for smaller sizes of sodium vapor arc lamps, up to about 150 watts, that the length and bore of the arc discharge tube are sufficiently small and the tube consequently sufficiently light-weight to permit dispensing with support for the upper end of the support frame. This obviates the need for a dimple at the dome of the outer envelope and the clamps or clips for engaging it, as well as for multiple lateral leaf springs, thereby achieving significant cost savings. However, applicants have found that when the upper end of the support frame is left unsupported the lamp is susceptible to breakage when subjected to severe mechanical shock. A typical commercial acceptance shock test involves dropping a standard carton of lamps to a hard surface from a height of two feet; successively on the top, bottom and two adjacent sides of the carton. Failure of the lamps frequently occurs, applicants have found, because movement of the support frame causes the inlead conductor to which it is affixed in the base assembly of the outer glass envelope to break the re-entrant stem which seals the envelope.